1. Field of Invention
The present invention relates to a rubber composition for side wall of tires and more particularly to the improvement of the properties thereof in respect to bare spots, crack resistance, vulcanization-reverse and so forth.
2. Prior Art
General-purpose rubber polymers such as natural rubber (ab. NR), butadiene rubber (ab. BR) or general-purpose styrene-butadiene rubber (ab. SBR) have been conventionally used as the rubber polymers for a side wall of a tire. For preventing bare spots from being formed on a surface of a side wall, inorganic powder paste, e.g., carbon black paste comprising carbon blacks, volatile component and a small amount of binder is applied on a side wall portion in order to make it easy for the rubber polymers of a green tire to flow smoothly along the inner face of a metal mold without partially adhering thereto in the course of vulcanization and thereby not to entrap the air between the mold and the green tire but to lead the air to a vent hole so as to exhaust outside. The bare spots are caused by some factors such as vulcanizing speed, flowability of rubber polymer with a continued deformation under a high temperature and a load namely, flow behavior, smoothness of green tire surface and so forth.
As described above, bare spots are restrained in prior arts, by applying a carbon paste on a green tire to put an inorganic powder as an inadhesive lubricant between the inside of a metal mold and a rubber polymer and also on the other hand by making a smooth surface of a green rubber a little irregular enough for the air to be exhausted outside of a metal mold at an earlier stage of molding. But said carbon paste is not preferable because of occurrence of cracks on the surface of a side wall easily caused by ozone attack.